The present invention is concerned with a process of removing cadmium ions from aqueous, acid, especially P.sub.2 O.sub.5 containing solutions by liqud-liquid extractions with the acid of alkyl amine salts of the halogen-hydracids that are dissolved in an inert organic solvent.
Cadmium is a heavy metal the spreading of which is to be reduced. Crude phosphate ores used for manufacturing phosphate-containing fertilizers, contain cadmium. By the digestion of the crude phosphates with acid, the cadmium, in the form of an easily soluble salt, is passed into the generated phosphoric acid, and in the further processing thereof into fertilizers. A certain amount of cadmium is thereby supplied to the soil. Through intensive fertilizing for years, the cadmium content in the soil increases. To prevent a further increase in the soil, it will be imperative to use only such fertilizers containing no or almost no heavy metal.
There are differences in the cadmium content of the individual crude phosphate ores and, hence, in the cadmium content of the phosphoric acids produced therefrom. Efforts have been taken to reduce the cadmium content in the crude phosphoric acids produced from crude phosphate ores of high cadmium content. However, considerable difficulties have been encountered with the removal of cadmium from a crude phosphate acid. Cadmium salts exhibit a very high solubility, especially in acids of higher concentration.
Cadmium, usually, is precipitated from weakly acid aqueous solutions by treatment with hydrogen sulfides, as an insoluble sulfide. According to the statements in Gmelin's Handbuch der organischen Chemie, System No. 9, part B, p. 100, cadmium can be precipitated as an insoluble sulfide, provided that the acidity of the solution does not exceed an HCl amount of 1.38 normal, i.e. that the ability of precipitating cadmium is limited to hydrochloric acid concentrations of up to 5%.
According to German Offenlegungsschrift No. 24 22 902 it is possible to precipitate from phosphoric acids of higher concentrations having a P.sub.2 O.sub.5 content of in excess of 45 percent by weight, under a pressure of up to 50 atu (atmospheric excess pressure), and a dwelling time of up to 3 hours, cadmium from phosphoric acid. The examples show that a residual value of between 10 and 20%, based on the starting value of the cadmium, will remain in the acid. The process as disclosed by German Offenlegungsschrift No. 24 22 902 results in a reduction rather than in a removal of cadmium from the phosphoric acid. Apart from the unsatisfactory result of the described procedure, the process itself requires a high technical expenditure, as the reaction takes place in pressure vessels suitable for agitation.
Also, it is possible for cadmium to be removed from crude phosphoric acid by extraction with organic solvents. Technical possibilities for this are shown by the multiplicity of prior known processes for purifying phosphoric acid. In those processes pure phosphoric acid is recovered from which, in addition to cadmium also all other cations have been removed from the phosphoric acid. The acids in terms of purity are comparable with that of the thermal phosphoric acid. Owing to the high requirements placed upon quality, those processes are complex and very cost-intensive. The recovered pure acids for reasons of costs, in general, are not suitable for use in the manufacture of fertilizers. The object resides in providing a simple process permitting the removal from crude phosphoric acids of commercial concentrations or from other acid, especially P.sub.2 O.sub.5 containing solutions the cadmium contained therein without substantially changing the quality level of the acid, i.e. the content of other cations. The process should permit, if possible, the selective removal of cadmium from the solutions to be treated.